Oxygen isotope fractionation between calcite and fluid as a function of growth rate and temperature: An in situ study

The O-18 content of carbonates (expressed as delta O-18) is widely used in paleothermometry, yet it is clear from comparison of experimental results and natural carbonate samples that chemical (isotopic) equilibrium is not always achieved in nature. This observation underscores the importance of exploring possible effects of growth rate on isotopic fractionation, which is the focus of this study. In situ Secondary Ion Mass Spectrometry (SIMS) analyses of delta O-18 were performed on single crystals of experimentally grown calcite. The change in growth rate (V) over time within each crystal was monitored by addition of multiple rare earth element (REE) spikes into the fluid from which the calcite grew. The values of delta O-18 in bulk calcites and experimental fluids were measured by stable isotope ratio mass spectrometers. The average SIMS delta O-18 overlaps with those determined by conventional mass spectrometry within analytical uncertainty. The fractionation factor expressed as Delta O-18 approaches its equilibrium value in slowly grown calcites (V<0.02 nm/s) and decreases by 1.5 parts per thousand with increasing growth rate from 10(-3) to 0.34 nm/s. Here Delta O-18=10(3).ln(alpha O-18), and alpha O-18=(O-18/O-16)calcite(O-18/O-16)(fluid). Our results provide the first in situ evidence that under equilibrium conditions O-18 may be depleted in the near-surface region of calcite relative to the bulk crystal lattice, and that this O-18-depleted zone can be "captured" during rapid crystal growth. Crystal growth rate is therefore a potentially important consideration when using delta O-18 in natural carbonates as a proxy for ocean and terrestrial temperature. (C) 2012 Elsevier B.V. All rights reserved.